1. Field of the Invention
The present invention relates to data communication means and methods. Specifically, the invention relates to apparatus, methods, and systems for characterizing a communications channel to improve data throughput.
2. Description of the Related Art
Communication channels bear information between a sender and receiver and facilitate a wide range of interaction and information transfers such as verbal conversations, text messaging, document transfer, transaction processing, and the like. While most modem communication is conducted digitally, the lowest levels of communication generally involve generating and detecting units of information called symbols using analog means. Often such symbols are communicated via a dispersive and noisy media that degrades the signal as it is propagated. The ability to properly receive a transmitted symbol requires allocating sufficient bandwidth and energy to maintain the integrity of the symbol along the entire propagation path.
FIG. 1 is a block diagram depicting certain aspects of a typical prior art communications system 100. As depicted, the prior art communications system 100 includes an encoder 120 and a decoder 140 equipped with a characterization module 150 such as a signal-to-noise estimator. The encoder 120 receives a data stream 110 which is encoded for transmission over a dispersive and noisy communications channel 130. The encoded data stream is decoded to reconstruct the data stream 110. Due to degradation of the encoded data stream and the resulting communication errors, reconstruction of the data stream 110 may require retransmission of portions of the data stream 110. Such retransmissions are typically expensive and significantly reduce the throughput attained on the communications channel 130.
According to the well known information theorem of Shannon, the maximum data throughput that may be attained on a communications channel 130 is proportional to the bandwidth of the medium and the signal-to-noise ratio (SNR) experienced by the receiver. To optimize data throughput, the communications system 100 may be equipped with a characterization module 150 that characterizes a communication channel's attributes such as bandwidth and signal-to-noise ratio in order to maximize information throughput on the communications channel 130.
In the depicted embodiment, the characterization module 150 monitors the decoding process within the decoder 140, and provides characterization data 160 to the encoder 120 to optimize the encoding process for the communications channel 130. Often such characterization data is initially collected during a training session in which known signals are transmitted across a communications channel. By using the known signal as a reference, and comparing it to what is received, the receiving side characterizes the channel and noise conditions. Once a channel has been characterized, a calculation may be conducted to estimate the number of bits or symbols that can be successfully transmitted from the sender to the receiver. In many systems, channels are periodically or continuously characterized during operation to adjust to changing conditions channel conditions.
Accurate characterization of the communications channel 130, particularly the signal-to-noise ratio of the channel, is essential to maximizing the actual throughput of information. For example, if the actual noise is less than the estimated noise, some of the channel capacity remains unused. On the other hand, if the actual noise is greater than the
estimated noise, the communication channel would be incapable of bearing the amount of information that is sent resulting in errors and retransmissions.
In the depicted arrangement, a single transmission channel is depicted. Often, data communication is partitioned into multiple subchannels and assembled at the receiving end. For example, communication between central offices and subscribers is often conducted using multi-tone encoding techniques in which the available spectrum of the channel is divided into frequency sub-bands or tones and each sub-band or tone carries information in parallel with the other sub-bands or tones.
With multi-tone encoding techniques, the ability to characterize each sub-band and adjust to changing channel conditions may significantly increase the throughput attainable on the transmission medium. Unfortunately, many mediums such as subscriber lines connected to central offices may experience impulse noises (typically shorter than 200 microseconds). Such impulse noise may occur frequently enough to cause transmission errors yet infrequently enough that such impulses are averaged out and not accurately captured in the characterization data. The result is substantially reduced performance due to the poor characterization data.
To address the issue of poor characterization data in such systems, a relatively large noise margin may be allocated to each subchannel or tone. Under such a scenario, a transmission error occurs only when the total noise (including the impulse noise) exceeds the noise margin. However, such a strategy is less than optimal in that impulse noise is typically not equal in all sub-bands (i.e. white in spectral content) and the amount of needed noise margin may vary for each sub-band or tone. In other words, those tones that experience greater impulse noise should be allocated a larger noise margin than those tones that experience less impulse noise.
Given the aforementioned issues and challenges related to data communication and the shortcomings of currently available solutions, a need exists for an apparatus, method, and system for characterizing a communications channel and improving data throughput. Beneficially, such an apparatus, method, and system would account for impulse noise in the characterization process and facilitate increased data throughput on a communications channel in an efficient effective manner.